About four years ago, Janne Lempe, then a graduate student at the Max
Planck Institute for Developmental Biology in Tübingen, was hard at work on
a series of experiments on the genetic regulation of flowering time in
Arabidopsis thaliana. She crossed two strains, chosen for
their difference in flowering time, and set off to the greenhouse to leave
the seeds to grow.

Researchers generally raise
Arabidopsis at 23° C, but with the lab's main greenhouse
full, Lempe plunked her experiment into the 16° C greenhouse next door. A few
weeks on, her seedlings had produced stunted, stumpy, deformed versions of
the hearty white-flowered weeds she was expecting. Her plants were not only
unusually small, but their leaves were also covered in necrotic brown
spots, and they'd failed to flower.

Lempe's two starting strains, UK1 and UK3, were perfectly normal.
So why would their offspring be sick? Right away, says Detlef Weigel, an
evolutionary geneticist who supervised Lempe's doctoral research, "I
thought this might have some bearing on speciation." Perhaps genes from one
strain were incompatible with those in the other, creating in the offspring
a gene-flow barrier, which is the first step to the evolution of a new
species. Researchers have mapped genes in
Drosophila that keep two species apart, but such studies
can't resolve whether genetic incompatibilities drove speciation, or
instead accumulated after the species diverged. Weigel wondered whether
Lempe's sick hybrids might shine light on the process from the other side, by
showing how such incompatibilities could arise in a single species.

Lempe was surprised to find that her plants recovered, and even
flowered, when she transferred them from the cooler greenhouse to the
warmer one. (It's unclear why, but it's not unusual for growth effects to be
temperature sensitive.) So she took the seeds and established a homozygous
strain from the sick crosses. The results suggested that the interaction of
two genes, an allele from each parent strain, had caused the necrosis in the
plants.

When Weigel showed pictures of stunted UK1/UK3 crosses to Jeff
Dangl, a plant immunologist at the University of North Carolina, Chapel
Hill, Dangl pegged the problem at a single glance. "I said, 'that's
R-genes,'" he recalls. R-genes, one of the cornerstones of plant immunity,
are activated in response to specific pathogens; he had seen similarly
necrotic specimens in which these genes were overexpressed. Lempe's
microarray studies had shown that most of the differentially expressed
genes in the crosses were involved in pathogen resistance and immunity. The
duo plotted a collaboration.

Dangl's instinct proved correct. Kirsten Bomblies, a postdoc who
joined Weigel's lab a few months on, mapped one of the incompatible alleles
to an R-gene (
PLoS Biol, 5:1962-72, 2007). The second allele is still
mysterious, but Bomblies suspects that it's an R-gene, since several genes
on the affected locus are R-genes; unpublished work on other necrotic
crosses has also pointed to immune genes. Moreover, the hybrid plants are
more resistant than normal to pathogens, suggesting that a revved up immune
system - the plant world's version of autoimmunity - was causing the
necrotic phenotype.

Indeed, the literature on crop breeding is replete with
descriptions of two normal strains producing a stunted hybrid. In a paper
published in 1929, a researcher suggested that the sickness looked like an
immune response, proposing that plants must therefore have circulating
antibodies. His idea was ridiculed, and it was another 30 years before
breeders began again noting a possible link between hybrid necrosis and a
pathogen response, Bomblies says. The key now, she notes, is to track the
effect in the wild and in other species. So far she has performed 500 or so
hybrid crosses from Arabidopsis strains collected throughout Europe;
about 2% of those crosses are necrotic. "As far as I know," she says wryly,
"it's the biggest intercrossing scheme of natural strains in
Arabidopsis."

Weigel is quick to point out that the researchers may not be
witnessing a speciation event. "We cannot predict that these things are
going to turn into new species; in fact, it's very unlikely," he says, adding
that there must be many cases in which the process starts but is never
completed. Still, he insists, "it's important as a model."

"I think that's fair," says John Willis, an evolutionary
geneticist at Duke University who works on
Mimulus, commonly called monkey-flower. What's
interesting, he notes, is that the effect seems to have evolved "as a
byproduct of adaptive natural selection acting on these genes for
completely other purposes." His group has found sterile hybrids of
Mimulus strains, and he and Bomblies recently discussed
plans to study the genetics.

"A lot of people have very strong feelings on how this is going to play
out," says Weigel. "I just say, 'I don't know, we'll see.'"